As RF band filters and IF band filters for mobile communication equipment such as mobile phones and surface acoustic wave (SAW) filters are widely used. The SAW filter has a simple structure in which only comb-shaped electrodes are formed of aluminum or the like on a piezoelectric substrate, such as of lithium tantalate crystal, and has advantages such as a sharp cut-off characteristic, small size and weight, and low cost. However, along with the development of mobile communication equipment, the demands for the cut-off characteristic and the reduction in size and height become increasingly severe year after year. Therefore, it is difficult to realize such demands for SAW filters.
Elastic boundary wave devices, which utilize waves propagating along the boundary between different media, may provide a sharp cut-off characteristic because they have a small frequency-temperature coefficient. The elastic boundary wave device may be reduced in size and height because it requires no hermetic seal. The elastic boundary wave device is expected to become a substitute for the SAW filter and is under development.
Elastic boundary wave devices discussed in JP-A-2006-279609 and JP-A-2008-67289 have at least one dielectric layer stacked on comb-shaped electrodes formed on a piezoelectric substrate.